---
title: "Need for parallel operation and dark lamp method"
subject: "Electric Machines II"
chapter: 1
key: "semester_5/electric_machines_ii:parallel-operation-need-dark-lamp"
source: https://www.bctengineer.com/q/semester_5/electric_machines_ii/parallel-operation-need-dark-lamp
site: BCT Engineering
---

# Need for parallel operation and dark lamp method

Electric Machines II · Chapter 1: Three Phase Synchronous Generator

- Asked in: 2082 Baisakh, 6 marks
- Video: [all dark lamp method of synchronization | two bright one dark lamp method | alternator generator](https://www.youtube.com/watch?v=3OO2HZ1UzKc) — Mind of Engineer 
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/electric_machines_ii/parallel-operation-need-dark-lamp
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machines_ii/chapter-1

## Question

Why we need to perform parallel operation of two alternators? Explain dark lamp method of synchronizing an alternator with the Busbar.

## Short answer

Parallel operation ensures continuity of supply, allows efficient loading of machines, permits maintenance without shutdown, and enables easy expansion. In the dark lamp method, three lamps connect across the open synchronizing switch. When all lamps flicker together and become dark simultaneously, voltages are equal and in phase; the switch is closed at mid-dark period.

## Answer

#### Need for parallel operation
Generating stations use several alternators connected in parallel to common bus-bars instead of one large machine because:

1. **Continuity of supply**: if one machine fails or needs maintenance, others keep supplying the load.
2. **Efficiency**: machines are most efficient near full load; units are switched in or out as load varies, so each runs near full load.
3. **Maintenance and repair** can be done one unit at a time without shutting down the station.
4. **Future expansion**: new units can be added as demand grows.
5. **Reserve capacity**: only a small spare unit is needed instead of a spare of the full station capacity.
6. Interconnected grids require all generators to run in parallel.

#### Dark lamp method of synchronising
Three lamps are connected across the open synchronising switch, each between a bus-bar phase and the same phase of the incoming machine (R–R', Y–Y', B–B').

```text
 Bus-bars      R         Y         B
               |         |         |
              (L1)      (L2)      (L3)
               |         |         |
 Incoming      R'        Y'        B'
 alternator    |_________|_________|
```

**Procedure**
1. Run the incoming alternator up to about synchronous speed by its prime mover.
2. Adjust its field current until its voltage equals the bus voltage (voltmeters).
3. Observe the lamps. If they flicker **together** (all bright, all dark at the same time), the phase sequence is correct; if they glow one after another, interchange two leads of the incoming machine.
4. The flicker rate equals the frequency difference; adjust the speed until flickering is very slow.
5. Close the switch in the **middle of the dark period** of all three lamps; then the voltages are equal and in phase.
6. Increase prime-mover input to make the machine take active load; adjust excitation to share reactive load.

**Limitation:** lamps are dark over a range of voltage (up to about 1/3 of rated), so the exact moment is uncertain; a voltmeter across a lamp or a synchroscope gives better accuracy.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why are several alternators operated in parallel instead of one large one?
   A. To increase the frequency
   B. To reduce the terminal voltage
   C. To avoid the use of prime movers
   D. Continuity of supply, efficient loading, maintenance and expansion flexibility
   Answer: D. Parallel units keep supply during faults, run near full load, allow maintenance and expansion. (Key point: Need)

2. In the dark lamp method, lamps glowing one after another means
   A. phase sequence is wrong; interchange two leads
   B. frequency is exactly equal
   C. voltages are equal and in phase
   D. the machine can be closed in
   Answer: A. Correct sequence gives simultaneous flicker. (Key point: Sequence check)

3. The rate of lamp flicker equals
   A. the sum f1 + f2
   B. the frequency difference f1 - f2
   C. the voltage difference
   D. the phase sequence
   Answer: B. Slow flicker means frequencies nearly equal. (Key point: Frequency)

4. The switch is closed
   A. when lamps are brightest
   B. when lamps start flickering fastest
   C. in the middle of the dark period of all lamps
   D. after the machine takes load
   Answer: C. Dark means voltage across the switch is zero: equal and in phase. (Key point: Closing)

## Related in Electric Machines II

- [Parallel operation conditions and dark lamp synchronizing](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Aparallel-conditions-dark-lamp) — Electric Machines II, chapter 1, asked 3×
- [Parallel operation conditions and three lamp method](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Aparallel-conditions-three-lamp) — Electric Machines II, chapter 1, asked 3×

## Related in other subjects

- [Synchronizing alternators by dark lamp method](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Async-alternators-dark-lamp) — Electrical Machines, chapter 6
- [Criteria for synchronizing two alternators](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Asynchronizing-two-alternators-criteria) — Electrical Machine, chapter 6
- [Criteria for synchronizing two alternators](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Async-alternators-synchronizing-criteria) — Electrical Machines, chapter 6
- [Need and conditions for parallel operation of transformers](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Atransformer-parallel-operation-conditions) — Electrical Machines I, chapter 2